The present invention generally relates to a resilient mount for a shaft and, more specifically, to a resilient mount having uniform stiffness to achieve and maintain controlled stiffness of the rotor shaft support system.
In the field of rotating machinery, new designs are pushing towards more efficient, higher power, higher speed flexible rotating shaft assemblies to form part of highly competitive products, such as turbine engines, turbo machinery, electric motors and generators. In high speed machinery such as these, potentially large centrifugal forces can be imposed by the rotating components operating at high speeds. These components must be precisely balanced to avoid vibration which may lead to deviation of the shaft axis from its intended axis of rotation. Practically achieving, and maintaining, this precision balance can be difficult due to variations in the manufacture and assembly process, particularly for electric motors and generators.
The amplitudes of vibrations resulting from out of balance can be significant if the rotational speed reaches its resonance speed, or a multiple of its resonant speed. Such speeds are generally referred to as “critical speeds.” Critical speeds and machine response is a function of the mass, the shaft, bearings, housing and interface. Typical rotating machines, especially aerospace, employ rolling element bearings which have very high stiffness and provide little damping. If an unbalanced shaft is rotating for prolonged periods of time near one if its critical speeds, it may be damaged, even catastrophically.
U.S. Pat. No. 3,704,922, issued to Kleinschmidt et al., describes an intermediate bearing assembly and its supporting structures for an automotive application. The '922 patent is drawn to a design to secure an automotive drive shaft radially but still allow axial movement with an elastomeric tire. The design of this patent does not migrate the critical speeds of a high speed rotating shaft outside of the operating speed range.
U.S. Pat. No. 5,033,875, issued to Moulinet, describes a vibration reduction system for use in the automotive industry, especially for relatively low speed motor vehicle transmission shafts. The vibration reduction is achieved by using an elastomer ring damper between the bearing outer race and the support structure. This ring damps and reduces the vibration and prevents it from being transmitted to the chassis and passenger cell of the vehicle. This patent, however, is not designed to migrate the critical speeds of a high speed rotating shaft outside the operating speed range. The elastomeric ring disclosed in the '875 patent does not have a uniform cross-section, thus requiring specific manufacture and design considerations.
U.S. Pat. No. 7,052,183, issued to Chen et al., describes a centering ring having a specific geometric cross section. The centering ring only provides for an average stiffness, taking all the different cross sectional areas into account. Moreover, the resonant frequencies and critical speeds are more difficult to predict due to the variable stiffness/geometric cross section of the centering ring. Finally, having a variable geometric cross section, the centering ring of the '183 patent may require specific manufacturing costs and time.
There are existing shaft mounting systems to migrate the critical speeds of the rotating machinery outside the operating speed range. In particular, the use of mechanical springs usually consist of a band of material with a number of equally spaced support bumpers on the ID and OD. Problems with current rotating machinery resilient mount systems include 1) they are difficult to analyze accurately because the stiffness of the spring varies depending on the direction of load application; 2) they are prone to wear and fretting because of the relatively small portions of the centering rings in contact with either the bearing or the housing; 3) they are relatively difficult and expensive to machine due to their design; and 4) they addition of more bumpers to increase the spring stiffness complicates the machining further and increases cost.
As can be seen, there is a need for an improved mounting system for high speed rotating shafts that is easily machined, has a constant stiffness and provides the ability to migrate the rotor critical speeds out of the operating speed range.